1. Field of the Invention
The invention relates generally to the field of video decoding. More particularly, it relates to an improvement in processing motion vectors in the downsampling of digitized video data.
2. Description of the Related Art
Moving pictures can now be stored, transmitted, retrieved and processed in digital form, allowing the benefits of computer technology to be extended to the technology of moving pictures. However, the extensive amount of information present in moving pictures creates a very large amount of digital data, resulting in excessive storage and/or transmission requirements. Various forms of compression have been developed to convert the source images to a reduced-data format for storage and/or transmission. For display, this reduced data is reconverted to destination images that are equivalent to the source images.
Digital compression methods can also be combined with other conversion processes. Digital moving pictures are frequently recorded at the highest resolution that is currently feasible, but must be converted to lower resolution for display on the display devices that are most common, and for picture-in-picture applications.
Conversion techniques generally divide the image into subunits called macroblocks (typically 16xc3x9716 pixels) and then operate on each macroblock individually. Motion vectors are used to describe motion within a macroblock from one frame to the next. A motion vector can contain several components, with each component denoting the horizontal and vertical displacement from the current target pixel location to find the source pixel value.
Digital sequences must sometimes be coded as a mixture of sequential pictures, in which the entire picture is generated with sequential scan lines, and interlaced pictures, in which the even numbered rows of pixels (even field) are generated first, and the odd numbered rows of pixels lines (odd field) are placed between them. The decoder must handle such a mixture in a coded bitstream. The process of a decoder generating a sequential picture by predicting it from an interlaced picture is referred to as field frame processing.
All of the above concepts are well known in the art and are not further described herein.
Common algorithms are used for processing motion vectors in field frame processing. However, these algorithms operate on twice as many pixels as do the algorithms for other types of motion processing. This requires makers of video compression systems to include additional processing capability for field frame motion that is not used for any other type of motion processing. They must either include circuitry for processing twice as many pixels as are normally needed, or they must operate on only half the field frame pixels at a time and sum the results later, which reduces throughput and requires extra logic for processing the intermediate results.
FIG. 1 shows a vertical column (1xc3x978) of interlaced pixels from a reference picture that might be processed in field frame processing, with pixels A(0-3) representing the even field and pixels B(0-3) the odd field. With reference to these pixels, a conventional algorithm for calculating motion vectors in field frame processing is shown in FIG. 2, where filter coefficients h0, h1, h2 and h3 have predetermined constant values. Positions 0-7 represent the eight values that may define the amount of translation between the source pixel location and the destination pixel location. This process requires operating on eight source pixels for field frame motion processing, whereas the other types of motion processing can generate equivalent quality results while operating on only four pixels. FIGS. 3a and 3b show the logic for executing a conventional field frame motion vector process. In FIG. 3a, 8-input filter 21 can process all eight pixel values simultaneously and provide the results at output 28, but half of the complex filter capacity is unused for all operations except field frame motion vector processing. Alternatively, the 4-input filter 22 of FIG. 3b can be used to process the A pixels, store the results, process the B pixels, and then sum the two sets of results at output 29, but this cuts throughput significantly and requires additional logic in the form of input multiplexers 23, storage registers 25, and summation logic 26.
An embodiment of the invention is a method of generating motion vectors in a field frame image processing system. The method includes defining a column of pixels that has even rows alternating with odd rows in an interlaced video image. A predefined first set of pixels is selected from the even rows, the first set composed of less than all the pixels in the even rows. A predefined second set of pixels is selected from the odd rows, the second set composed of less than all the pixels in the odd rows. The first and second sets of pixels are operated on to produce a field frame motion vector.